1. Field of the Invention
The invention relates generally to nuclear waste storage systems and, more particularly, to a system for increasing the storage capacity of a water pool used for the storage of spent nuclear fuel rods.
2. Description of the Prior Art
Spent nuclear fuel rod assemblies, once removed from the nuclear reactor core, are normally stored in underwater storage areas in specifically designed storage racks for sufficient periods of time to permit the residual reactivity of the nuclear fuel contained therein to dissipate, or until the spent fuel rod assemblies are withdrawn from the pool for transportation to a spent fuel recovery facility. The fuel rods are metal pipes which are filled with nuclear fuel material and are typically 0.4-0.6 inches in diameter and 8 to 15 feet in length. Groups of 64, 128, or more such rods are confined within a fuel rod assembly while they are in a nuclear reactor and subsequently while they are stored in the underwater storage area. It is a normal practice to provide empty spaces in the storage rack between the spent fuel rod assemblies to preclude any unattended temperature increase within the underwater storage area. When an underwater storage area has been filled with spent fuel rod assemblies in such a manner, there is no space remaining within the storage area for additional spent fuel rod assemblies.
Increasing the density of the material stored in the underwater storage area allows a greater amount of nuclear waste material to be stored in the limited available space of the underwater storage area. To accomplish this end, it has been proposed to withdraw the spent fuel rods from the assembly in which they have a normal spacing and to compact the spent fuel rods in a canister in which the fuel rods are compacted in tighter spaces. The more compact spent nuclear fuel rods do not present the same tendency for temperature increase and can be stored in adjacent and contiguous spaces in a storage rack, thereby increasing the storage capacity of an existing storage rack three-fold.
A factor limiting the increase of the storage capacity in the underwater storage areas is the possibility that the loading capacity of the floor (usually concrete) of the storage area may be exceeded by such increased storage. Additionally, because it is essential to maintain the spent fuel rods under at least 10 feet of water at all times, including those times when the spent fuel rods are being transferred from their fuel rod assemblies into a compacted fuel rod canister, typical underwater storage areas are about 45 feet deep. This water further increases the loading exerted on the floor. The floor loading, therefore, is the weight of the 45 foot deep volume of water in the storage area and the contained storage rack and spent fuel rod canister.
It has been further proposed to position buoyant chambers above the storage rack containing spent fuel rod containers, and attaching the chambers thereto. See copending U.S. Pat. Application Ser. No. 660,477 filed on Dec. 7, 1984 by W. J. Wachter.
It is possible, therefore, to increase the storage capacity of the underwater storage areas through use of such buoyant chambers, while not endangering the structural integrity of the storage area. A further increase in the density of the materials stored in the underwater storage area, would allow additional nuclear waste material to be stored in the existing storage area.
It is, therefore, an object of the present invention to provide a system for maximizing the packing density of spent fuel rod canisters while minimizing the load which is applied to the floor of an underwater storage area for spent nuclear fuel rod storage.
It is a further object of the present invention to provide a spent fuel rod canister which requires the minimum amount of support from a storage rack, thereby maximizing the available storage capacity of an underwater storage area.